Hi, Null,

I can't answer your important question on the valve....never owned one. Others on the forum do, however, so this will bump you back to the top for a response.

Question #1 is somewhat easier. Don't remove that cap with the pump running!! Seriously, it may be a spare for some future purpose or , more likely, it was for something that has since been removed. If your plumbing appears to be functioning normally, I'd keep thinking about it for a while and I'll bet you'll figure out it's purpose. You might even shut the pump off, take off the cap, and turn the pump back on....be prepared to shut it back off!!